This invention relates generally to a method and apparatus for CT imaging and other radiation imaging systems and, more particularly, to determining the focal spot position of an imaging system, providing a real-time feedback input to system for correction or calibration of focal spot induced errors.
In at least some “computed tomography” (CT) imaging system configurations, an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system and generally referred to as an “imaging plane”. The x-ray beam passes through an object being imaged, such as a patient. The beam, after being attenuated by the object, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at a detector array is dependent upon the attenuation of the x-ray beam by the object. Each detector element of the array produces a separate electrical signal that is a measurement of the beam attenuation at the detector location. The attenuation measurements from all the detectors are acquired separately to produce a transmission profile.
In third generation CT systems, the x-ray source and the detector array are rotated with a gantry within the imaging plane and around the object to be imaged, so the angle at which the x-ray beam intersects the object constantly changes. X-ray sources typically include x-ray tubes, which emit the x-ray beam at a focal spot. X-ray detectors typically include a collimator for collimating x-ray beams received at the detector, a scintillator adjacent the collimator, and photodetectors adjacent to the scintillator. A group of x-ray attenuation measurements, i.e., projection data, from the detector array at one gantry angle is referred to as a “view”. A “scan” of the object comprises a set of views made at different gantry angles, or view angles, during one revolution of the x-ray source and detector.
In the third generation CT scanners, tube focal spot has been regarded as a stationary x-ray source with respect to x-ray detector. However, tube focal spot motion relative to detector is unavoidable due to x-ray generation mechanism and tremendous stress induced by extreme thermal and gravitational load operating conditions.
It is desirable to have a stationary tube focal spot. Otherwise, detector collimator plate aiming accuracy has to be maintained so that detector would have a linear response with respect to any focal spot motion in directions, which are perpendicular to collimator plate. In some circumstances, the required tolerance on deviation from design nominal is less than 1 micron, well beyond capabilities of existing manufacturing technologies.
To mitigate difficulty of maintaining collimator plate aiming accuracy, many manufacturers adopted a method of purposely misaligning (skewing) plates to focal spot. However, this would have other adverse affects. Even with ‘misaligning’ design, plate-aiming accuracy still remains at the limit of current manufacturing technology. Significant yield loss occurs just for collimator plate flatness (plate bow) requirement. For 40 mm Z-coverage detector collimator, plate yield due to plate flatness specification may be less than 50%.
To maintain image quality on low contrast detectability, CT number accuracy and artifact minimization, CT detector collimator scatter rejecting capability requirement increases with increasing scan coverage in the Z-axis. One option is to increase collimator aspect ratio by increasing plate height in the direction of x-ray projection. Plate height increase, however, presents a major challenge for detector design and manufacturing, since detector sensitivities to focal spot motion increases at same rate.
Tube focal spot motion induces not only beam position change at the detector, but also the X-ray incident angle change. Although beam position sensing and tracking provide adequate performance regarding dose saving and detector Z-axis non-uniform correction, it would be desirable to be able to measure focal spot position directly so that both beam position and incident angle can be captured. It has been discovered that x-ray beam incident angle, in addition to beam position, at the detector has significant impact on its performance. It is therefore seen to be desirable to measure the tube focal spot position with respect to detector.
If the position of the focal spot can be accurately measured in real time, control methods are known in the art to reposition the focal spot within the tube using magnetic or electrical fields, for example U.S. Pat. No. 5,550,889 or software correction algorithms can be employed during reconstruction to compensate for the undesired effects.